Intrinsic Chirality Properties of the Xenopus Egg Cortex and Left-Right Asymmetry

Mike Danilchik
Integrative Biosciences, School of Dentistry
Oregon Health & Science University
danilchi@ohsu.edu
Presented in the Embryo Physics Course, June 18 and June 25, 2009

Abstract

Presentation

/files/presentations/Danilchik2009a.pdf

Movies

Movie1.mov is a timelapse showing several normal Xenopus eggs go through fertilization, activation, surface contraction waves, and the first two cleavage cycles. At the beginning of the movie, eggs are held in random orientation because the egg surface is closely adherent to the surrounding vitelline (fertilization) envelope and a thick transparent layer of jelly. After fertilization, the egg surface becomes uncoupled from the vitelline envelope, and rights itself, pigmented animal pole up, because of buoyant density differences between the heavy yolk mass (in the unpigmented vegetal hemisphere) and the lighter, more fluid animal hemisphere. Fert to first cleavage: about 90 minutes. Second and third cleavages each 30 minutes long.

Several movies of Xenopus gastrulation & neurulation can be found at http://www.xenbase.org/xenbase/original/atlas/movies.html

MovieWK belongs to Weaver & Kimelman (2003) and can be launched from Development site: http://dev.biologists.org/content/vol130/issue22/images/data/5425/DC1/Movie3.mov

Movie2.mov, Movie3.mov, Movie4.mov: these three movies are first, second, and third cleavage cycles, respectively, of eggs exposed to inhibitor BDM (about 10 minutes before movie starts). The counterclockwise rotation of each active cleavage furrow is quite evident. Note also that the torsion only occurs while contractile ring is operating; in other words, earlier cleavage planes do not exhibit torsion.

[there is no Movie5 in current lecture]

Movie6.mov is a short confocal-microscope time lapse, following the motion of fluorescent actin that was injected into the precleavage egg. The optical section is a grazing section through the cleavage furrow, analogous to an elevation contour line following a valley. Shear of the cortical actin (actomyosin?) is evident across the width of the contractile ring the furrow. Rate of translocation between the two sides is about 35 microns/minute.

Movie7.mov shows several UNfertilized eggs exposed to BDM. The 5 eggs on the right are activated by pricking with a sharp microinjection needle (producing a small amount of leakage. Near the time that a normally fertilized egg would begin cleavage, these parthenogenetically activated eggs begin a continuous rotation, counterclockwise as usual, of the animal hemisphere relative to the vegetal hemisphere. Note that unactivated eggs (the three on the left) appear not to respond to the drug at all.

Movie8.mov is a field of Dendraster excentricus (sand dollar; oursin plat) eggs undergoing first cleavage in presence of BDM. Eggs were devitellinated (i.e., they lack a fertilization envelope) and a suspension of small carbon particles was sprinkled over them to show motion better. As cleavage begins, blastomeres rotate approximately 180 degrees relative to each other.

Movie9.mov shows the formation, extension, and eventual torsion exerted by a pseudofurrow that was induced by injecting active RhoA into a parthenogenetically activated egg (activated without sperm). Unlike normal furrows in fertilized embryos, pseudofurrows typically cannot deepen, and often bifurcate and eventually subside. This RhoA-induced pseudofurrow is unusual, in that it organizes cortical actomyosin into a single, long linear array. As seen in the time lapse, extensive shear (about 40 microns/min) is exerted across its extent.

Links

http://www.ohsu.edu/cellbio/faculty/faculty%20pages/danilchik.html

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